1. Field of the Invention
The present invention relates generally to controls for induction motors used in constant mass air flow, sometimes also called constant CFM (cubic feet per minute), applications.
2. Discussion of the Related Art
In modern heating systems, it is sometimes desirable to regulate the amount of air flow through the heat exchanger to a constant CFM volume. References addressing air flow control by a motor driven blower system are known to exist. The reader is referred to U.S. Pat. No. 5,736,823, to Norby et al.; and U.S. Pat. No. 5,202,951, to Doyle; for examples of the known art.
Generally the constant mass air flow blower controls of the known art may require fan speed sensing, motor current sensing, torque calculations, or some combination of the above which may make the systems expensive in terms of the sensing apparatus, mathematical processing power and the like. Such control systems may also incur time delays during control calculation.
Known examples of control systems in the art may otherwise rely on an assumption of a linear relationship between fan, or motor, speed and mass air flow. While a degree of linearity may be achieved in certain systems with expensive variable speed controllers such as a pulse width modulated (PWM) controller, the attendant cost may be prohibitive. Also, for readily available and inexpensive induction motor driven systems the above assumption of linearity does not hold true, and systems based on this assumption may not yield adequate control stability or performance with or without more expensive electronic motor controllers, resulting in operation which is not optimally smooth or quiet.
Therefore to solve the shortcomings of the known art, there is needed an inexpensive motor control system for induction motors utilized in constant mass air flow systems.
The present invention provides an inexpensive and reliable constant mass air flow controller for induction motor driven blower systems. The present system requires monitoring only of the blower, i.e. fan or motor, speed in conjunction with a motor controller which does not assume linearity of speed, motor control voltage and flow rate.
Instead, according to the present invention, the controller is provided with a look up table covering the operating range of the motor, which is accessible by the motor controller processor. The look up table contains a family of fairly straight curves for several motor speeds plotted against proportionality constants of air flow rate to fan speed on one axis and the control voltage settings on the other axis.
In order to select the proper motor control voltage setting, the motor controller compares the measured speed of the motor, or fan, of the blower (hereinafter referred to as just xe2x80x9cmotorxe2x80x9d or xe2x80x9cfanxe2x80x9d synonymously) against the control voltage setting to derive the proportionality constant known to give the proper mass air flow. The controller then derives the proper motor speed, or xe2x80x9cRPM setpointxe2x80x9d, to achieve the desired mass air flow. The excitation voltage is then increased or decreased to achieve the proper motor speed. The control voltage setting will sometimes also be referred to as a xe2x80x9ccontrol pointxe2x80x9d or xe2x80x9ccontrol settingxe2x80x9d since voltages may not be directly represented under the scheme of excitation used to control the motor, as will be understood by the person of ordinary skill in the art.
A cascaded control loop is used for the motor controller of the invention to attain a constant mass air flow. The outer loop of the cascade control has an input of the selected constant CFM rate and an output of the RPM setpoint to the inner control loop. The inner control loop has an input of the RPM setpoint and outputs to the outer loop the control voltage setting when the RPM setpoint is achieved.
The outer loop uses the measurement of the motor speed and reported control voltage at that speed to derive a proportionality constant of the system operation for that motor speed. The proportionality constant contains the air flow information necessary to select the next RPM setpoint for operation of the motor to achieve the selected constant CFM mass air flow. If necessary, a new RPM setpoint is selected, and the control voltage adjusted, to increase or decrease fan speed to achieve the desired air flow; with a rechecking of the proportionality constant for the new fan speed attained under the given system load. Iterative adjustment of the RPM setpoint is performed until the desired mass air flow is reached.
The system relies on the fact that for a constant system load, flow rate is proportional to fan speed. Because the system load for a blower motor generally remains constant and changes by a significant amount only occasionally, the system need only monitor the motor rotational speed, which is a function of the system load, and check the motor speed and voltage control point to derive the proportionality constant. The selected CFM value is then divided by the proportionality constant and used to select the next RPM setpoint for the motor and the control voltage is changed accordingly. During most periods of use little adjustment is needed, so the motor controller may monitor speed changes at a long time constant, or may operate with a lower allowable system adjustment, or xe2x80x9cgainxe2x80x9d, to make sure small transients in motor speed do not affect system stability. When the system load changes significantly, thereby causing significant fan speed change or control voltage adjustments, a short time constant for the control loop is used whereby the RPM setpoint and control voltage adjustment occur more frequently until the desired constant mass air flow rate is again attained at the new load level under a new control voltage.
By using the cascade control loop algorithm of the present invention minimal hardware is required since the cascade control is merely a software implementation. Also, direct control of the motor speed removes speed variations due to drifts in motor temperature, line voltage, air temperature, etc. The lookup table storage for motor/fan characteristics of the present invention promotes efficiency of operation since the family of control curves tends to be close to a set of straight lines. The addition of the adaptive control in the outer loop of the control for the present invention will provide very stable motor control that is responsive to system load variations.